1. Purpose
This memo explains how to calculate maximum demand for commercial buildings under AS/NZS 3000:2018. You need accurate maximum demand figures to size consumer mains, submains, distribution boards, and protective devices.
Get the numbers wrong and you end up with undersized cables, nuisance tripping, or overloaded switchboards. Get them right and the installation runs safely within its rated capacity from day one.
2. What Maximum Demand Means in Practice
Maximum demand is the highest electrical load your installation will draw at any point during normal operation. It drives every upstream sizing decision - from the main switchboard bus bars down to the consumer mains cable.
AS/NZS 3000:2018 Clause 2.2.2 requires you to determine the maximum demand before you size any part of the installation. The standard gives you four ways to do this:
- Calculation - Apply the diversity factors from Appendix C tables to each load group. This is the most common method for new commercial fit-outs.
- Assessment - Use engineering judgment when load patterns are complex, intermittent, or do not fit standard diversity factors. You must document your reasoning.
- Measurement - Record the actual peak demand over a 30-minute sustained period using a maximum demand indicator. This works best for existing buildings where you have metered data.
- Limitation - The maximum demand equals the fixed rating of the upstream protective device. This applies where supply authority constraints cap the available capacity.
For most new commercial projects, you will use the calculation method with Table C2 from Appendix C. Assessment comes into play on complex sites - think data centres, hospitals, or buildings with large intermittent loads like commercial kitchens.
3. How to Calculate Maximum Demand for Commercial Buildings
Step 1: List Every Load
Walk through the design drawings and list every connected load. Group them into the categories that Table C2 uses:
- Lighting - total connected wattage for all luminaires
- Socket outlets (GPOs) - count of 10 A and 15 A outlets
- HVAC equipment - each unit's nameplate rating in kW or amps
- Motors - nameplate full-load current for each motor
- Cooking equipment - connected load for commercial kitchen appliances (refer to Table C5)
- Water heating - storage and instantaneous heaters at nameplate rating
- Lifts and escalators - nameplate motor ratings
- Special loads - UPS systems, server rooms, EV chargers, or any other dedicated equipment
Step 2: Apply Diversity Factors from Table C2
Table C2 in Appendix C covers non-domestic installations - offices, shops, factories, schools, hospitals, hotels, and churches. Each load group has its own diversity factor. Here are the key principles:
Lighting loads: Apply the connected lighting load in watts, convert to amps at 230 V single-phase or 400 V three-phase, then apply the Table C2 diversity factor. A typical office lighting load sits around 10–15 W/m². For a 500 m² office floor at 12 W/m², your connected lighting load is 6,000 W or 26 A at 230 V single-phase.
Socket outlets (GPOs): Table C2 provides demand allowances per socket outlet for commercial buildings. As a guide, for 10 A GPOs the standard allows 10 A for the first point plus 5 A for each group of 20 additional points (or part thereof). For 15 A outlets, allow 10 A for one or more on a circuit. Always confirm the exact values from Table C2 for your building classification - the figures vary by occupancy type.
Motors: Take the largest motor at 100% of its full-load current. All remaining motors go in at 50%. If you have a 15 kW motor (28 A at 400 V three-phase) and three 5.5 kW motors (10.5 A each), your motor demand is 28 A + (3 × 10.5 A × 0.50) = 43.75 A.
HVAC: Air conditioning units above 10 A use 75% of connected load as the demand allowance. A 30 kW packaged unit drawing 54 A at 400 V three-phase contributes 54 A × 0.75 = 40.5 A to maximum demand.
Water heating: Storage water heaters go in at 100% of nameplate rating. A 3.6 kW storage unit adds 15.7 A at 230 V.
Cooking equipment: Do not use nameplate ratings directly. Refer to Table C5 for the correct demand factors. Commercial cooking loads use a sliding scale - the more appliances connected, the lower the percentage applied to each additional unit.
Step 3: Add the Load Groups Together
Sum all the after-diversity demand figures from each load group. This total is your maximum demand in amps.
Step 4: Convert to kVA for Supply Authority Applications
Your distribution network service provider (DNSP) will want the figure in kVA:
Three-phase: kVA = 400 × A × 1.732 / 1,000
Worked Example - 800 m² Office Fit-Out (Three-Phase 400 V Supply)
| Load Group | Connected Load | Diversity Factor | After-Diversity Demand |
|---|---|---|---|
| Lighting (120 points, 9,600 W) | 13.9 A per phase | Per Table C2 | 10 A per phase |
| GPOs - 60 × 10 A outlets | 60 outlets | 10 A + 5 A × 2 | 20 A per phase |
| HVAC - 45 kW packaged unit (0.8 PF) | 81 A | 75% | 60.8 A |
| Motors - 1 × 7.5 kW, 2 × 2.2 kW | 14.3 A + 8.4 A | 100% + 50% | 18.5 A |
| Water heating - 1 × 3.6 kW | 5.2 A | 100% | 5.2 A |
| Server room - 10 kVA UPS | 14.4 A | 100% | 14.4 A |
| Total per phase | ~129 A per phase |
Maximum demand = 129 A × 400 V × 1.732 / 1,000 = 89 kVA
You would apply to your DNSP for a 100 kVA supply to allow headroom for future load growth.
4. Common Mistakes to Avoid
If you sum every nameplate in the building, you will massively oversize the supply. A 200 kVA connected load in an office might have a true maximum demand of 90–120 kVA after diversity.
Table C1 covers domestic installations only. Commercial and industrial buildings must use Table C2. The diversity factors differ significantly.
Commercial kitchens with multiple appliances need the sliding demand scale from Table C5. Using nameplate ratings for six deep fryers and four ovens will inflate your demand figure well beyond what the kitchen will actually draw.
While maximum demand is a sustained load calculation (30-minute window), you still need to check that your supply can handle motor starting inrush. A direct-on-line motor can draw 6–8 times its full-load current for a few seconds. This affects protective device selection, not maximum demand, but it catches people out.
Each DNSP publishes its own rules for connection applications. Some cap single-phase supplies at 63 A or 80 A. Others require three-phase above 15 kVA. Check these before you finalise your design - a recalculation after connection approval delays the project.
5. When to Use Assessment Instead of Calculation
The calculation method works for straightforward commercial fit-outs. But some buildings need the assessment method (AS/NZS 3000:2018 Clause 2.2.2):
- Data centres - IT loads run at near-constant demand with redundant power paths. Standard diversity factors do not reflect this. You need to assess based on the planned rack load density (typically 5–15 kW per rack) and the total number of racks.
- Hospitals - Critical care areas, theatres, and imaging suites have very different load profiles from general wards. Assessment lets you apply realistic factors to each zone.
- Buildings with large intermittent loads - Cold storage warehouses, commercial laundries, or manufacturing with batch processes. These loads cycle on and off, and standard tables cannot capture the pattern.
- Existing buildings with metered data - If you have 12 months of interval metering data, measurement gives you a more accurate maximum demand than any table. Use the highest 30-minute sustained demand from the metered record.
When you use assessment, document everything. Record the assumptions, the load profiles, the operating schedules, and the basis for each diversity factor you apply. This documentation forms part of the design record and must be available for inspection.
6. Key Takeaways
- 1AS/NZS 3000:2018 Section 2.2.2 requires you to determine maximum demand before sizing any part of the installation.
- 2Use Table C2 from Appendix C for commercial buildings - never Table C1.
- 3Apply diversity factors to each load group separately, then sum the after-diversity figures.
- 4Motors use 100% for the largest and 50% for all others. HVAC above 10 A uses 75%. Storage water heaters use 100%.
- 5Convert your total to kVA for DNSP supply applications. Allow 10–20% headroom for future growth.
- 6Use the assessment method for complex sites where standard diversity factors do not fit. Document your reasoning.
- 7Always check your DNSP's Service and Installation Rules before finalising the design - supply limits and connection requirements vary by network area.
Frequently Asked Questions
Use Table C2 from Appendix C for all non-domestic installations including offices, shops, factories, schools, hospitals, hotels, and churches. Table C1 covers domestic installations only and must not be used for commercial buildings.
AS/NZS 3000:2018 Clause 2.2.2 provides four methods: Calculation (apply diversity factors from Appendix C), Assessment (engineering judgment for complex loads), Measurement (30-minute sustained peak from metered data), and Limitation (fixed rating of upstream protective device).
For general motors, the largest motor goes in at 100% of full-load current and all remaining motors at 50%. Lift motors use a different scale: largest at 125%, next at 75%, and remainder at 50%.
For single-phase: kVA = 230 × Amps / 1,000. For three-phase: kVA = 400 × Amps × 1.732 / 1,000. Most DNSPs require the maximum demand figure in kVA for supply applications.
Use the assessment method for data centres, hospitals, buildings with large intermittent loads like cold storage or commercial laundries, and any installation where standard diversity factors from Table C2 do not reflect actual usage patterns. You must document all assumptions and reasoning.
This memo provides general guidance on maximum demand calculations under AS/NZS 3000:2018. It does not replace the standard itself. Always refer to the current edition of AS/NZS 3000 and your local DNSP requirements for project-specific compliance. CCC Engineering accepts no liability for designs based solely on this guidance without independent verification against the applicable standards.